1. Field of the Invention
The present invention provides a method for predicting lateral vibrations of the bottom hole assembly of a drillstring due to forward synchronous whirl at critical speeds and including using sensors to measure longitudinal and torsional movement at the top of the drillstring caused by such vibrations.
2. Background
Elongated drillstems or "drillstrings" used in drilling wells and the like often undergo damaging or destructive lateral vibrations of the bottom hole assembly. The bottom hole assembly of a conventional drillstring may comprise a predetermined set of coupled sections of heavy walled drill pipe or drill collars and selected spaced-apart bearing members commonly known as stabilizers. The bottom hole assembly may also comprise certain members such as drilling jars and members including measurement while drilling (MWD) instrumentation. Although the bottom hole assembly is often the stiffest portion of the drillstring, it is also that portion which is subjected to the greatest amount of compressive stress and, being farthest from the surface, is that portion of the drillstring for which vibrations in all modes are the most difficult to detect.
Efforts to analyze and deal with bottom hole assembly vibration are discussed in treatises entitled "Lateral Vibration: The Key to BHA Failure Analysis" by Robert F. Mitchell and Michael B. Allen, World Oil, March 1985, and in SPE papers numbers 14330 entitled "Field Measurements of Downhole Drillstring Vibrations" by S. F. Wolf, et al., September 1985, and SPE/IADC 18652, "Case Studies of the Bending Vibration and Whirling Motion of Drill Collars" by J. K. Vandiver, et al., March 1989. These publications all address the problem of analyzing and dealing with lateral vibration or bending of the bottom hole assembly and present suggested analysis techniques which are correlated with downhole measurement devices.
However, the use of downhole measurement devices is not always available or practical whereas the development of surface measurement devices and techniques for detecting downhole drillstring behavior has proven to be viable as indicated in U.S. Pat. No. 4,715,451 to Bsiesu, et al. and assigned to the assignee of the present invention.
Even with the advanced technique of determining drillstem behavior using the system and method of the '451 patent, the complex vibration signals sensed at the top of a drillstem during drilling operations has presented certain problems in detecting particular vibration modes which may be correlated with actual downhole behavior of the drillstem, particularly the bottom hole assembly (BHA). The terms "bottom hole assembly" and "BHA" are used interchangeably herein. Accordingly, there has been a strongly felt need to develop a method of detecting bottom hole assembly vibration conditions and predicting, for a particular drillstring, when certain unwanted vibrations will occur, such as those due to forward synchronous whirl at lateral critical frequencies or rotational speeds of the bottom hole assembly.
For example, it has been determined that, at a fundamental lateral critical frequency of a bottom hole assembly, when corresponding to the speed of rotation of the drillstem, deflection of the bottom hole assembly will occur which will cause impacting of the borehole wall by the bottom hole assembly components. Although the placement of stabilizers in a bottom hole assembly serves to journal the BHA in the wellbore, there is a slight eccentricity which occurs due to clearance between the stabilizer outside diameter and the borehole wall, nonlinearity of the wellbore and deflection of the bottom hole assembly due to axial loading on the drillstem. A typical bottom hole assembly may have a length of from 200 ft. to 1,000 ft. and the characteristics of each bottom hole assembly will vary substantially due to the make up of the members, such as the drill collars, stabilizers and other elements that may be present in the assembly. In accordance with the present invention, however, a unique method has been developed for determining potentially catastrophic downhole bending conditions of the bottom hole assembly while drilling including analysis of longitudinal and torsional vibrations of the drillstem at the surface.